gallivm: don't use vector selects with llvm 3.7
gallivm: don't use vector selects with llvm 3.7

llvm 3.7 sometimes simply miscompiles vector selects.
See https://bugs.freedesktop.org/show_bug.cgi?id=94972

This was fixed in llvm r249669
(https://llvm.org/bugs/show_bug.cgi?id=24532).

Reviewed-by: Jose Fonseca <61b5a29db2c0650f7c8510f93eece059c1b09320@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler